1. Field of the Invention
The present invention generally relates to printing machines for use with microprocessor-based computers and, more particularly, to page advancement control systems for use in such printers.
2. State of the Art
For printers that are used with personal computers and the like, it is important to provide print registration which is sufficiently accurate to avoid visually-observable defects. Avoidance of print mis-registrations is especially important in printers that print in multiple colors.
Printing errors of a cyclical nature are especially troublesome in printers such as inkjet printers that progressively print swaths of ink droplets across a page. Conventional inkjet printers include ones manufactured and sold by the Hewlett-Packard Company of Palo Alto, Calif. Examples of such printers include the Hewlett-Packard ThinkJet and DeskJet printers.
In operation, inkjet printers simultaneously print groups of ink droplets in vertical columns that travel horizontally across a page to form a swath. After a swath is printed, the page can be advanced and another swath can be printed. The printing may be textual or graphical. The spacing of ink droplets within a swath is controlled by the spacing of jets in the inkjet printhead and, in most inkjet printers, is highly accurate.
To avoid the creation of visually-discernable defects or "artifacts" when printing with inkjet printers, the separations between abutting swaths must be extremely precise. If there are gaps or overlaps between adjacent swaths, "banding" patterns may be created across a printed page. In fact, misalignment between adjacent swaths may be readily discerned for mis-registration distances as small as about one-half to one-tenth of a pixel's diameter at a density of about three hundred pixels per inch.
Print registration defects are also easily discerned in multi-color printing. For such printing, it is often necessary to precisely register primary color pixels with one another. In quantitative terms, pixel positioning for multi-color printing must be accurate to within about 0.1 mil to avoid visible defects and artifacts.
Theoretically, if a sheet is advanced in an inkjet printer by a distance exactly equal to one swath width, registration between printed swaths would be perfect. Heretofore, however, satisfactory accuracy has not been obtainable with conventional sheet advancement mechanisms in inkjet printers
Inaccuracies in the control of sheet advancement in printers such as inkjet printers are usually attributable to the mechanical nature of the sheet advancement devices. Conventionally, sheet advancement distance is controlled by detecting the angle of rotation of a shaft connected to a drive roller or wheel. The accuracy of sheet advancement control systems is limited, however, by the uncertain nature of the mechanical interfaces between drive rollers and sheets at the microscopic level. In particular, variations in the effective radius of a drive roller may cause variations in the distance that a sheet is driven--even if the angle of rotation of the roller is detected exactly. Also, sheet positioning errors can arise from motor load variations produced by friction and stiction. Still further, sheet positioning errors can arise from backlash or windup in the drive train for a drive roller.
To overcome inaccuracies is paper positioning in printers such as conventional tractor printers, it has been suggested to provide a closed-loop direct feedback control system that senses so-called demarcation markings on a printed page. The suggested system is further described in U.S. Pat. No. 4,734,868 issued Mar. 29, 1988. Such systems are not always satisfactory, however, because the demarcation markings may detract from the appearance of text or graphics printed on a page.